1. Field of the Invention
The invention relates generally to embossing technology. In particular, the invention relates to a releasing agent for embossing mold with high pattern density.
2. Background Art
U.S. Pat. No. 5,772,905 issued to Chou discloses a hot embossing technique, called nanoimprint lithography, for forming submicron patterns on a thin film carried on a substrate. The embossing technique involves forming a submicron pattern on a substrate by such methods as electron beam lithography (EBL) and subsequent etching processes so that the substrate can act as a mold. The mold with the submicron pattern is brought into contact with a thermoplastic polymer, e.g., polymethyl methacrylate (PMMA), carried on a surface of a substrate so that the pattern on the mold can be embossed on the polymer. During the embossing step, the polymer, the substrate, and the mold are heated to allow sufficient softening of the polymer relative to the mold. The mold is then urged at a molding pressure into the polymer so as to transfer the pattern on the mold to the polymer. After a period of time, the entire assembly is cooled below the glass transition temperature of the polymer, allowing the pattern transferred to the polymer to harden. Then, the mold is separated from the polymer.
Sticking when separating the mold from the polymer is a concern, especially if the mold has a high surface area. The main causes of sticking are chemical affinity and physical property at the interface between the mold and the polymer. Evolution of stresses between the polymer and mold during embossing also contribute to sticking. See, for example, H. Scheer et al., xe2x80x9cProblems of the Nanoimprinting Technique for Nanometer Scale Pattern Definition,xe2x80x9d Journal of Vacuum Science Technology, B 16 (1998), pp. 3917. In order to suppress such stress evolution, the polymer should have smaller thermal expansion and smaller pressure shrinkage at the embossing temperature and pressure ranges. Stress evolution may also be suppressed by changing the thermal cycle for the embossing process. However, finding the optimum cycle for the embossing process would require many experiments.
There are two general approaches for avoiding sticking between the mold and the polymer. The first approach involves modification of the polymer surface using an additive in the polymer. See, for example, S. Y. Chou et al., xe2x80x9cNanoimprint Lithography,xe2x80x9d Journal of Vacuum Science Technology, B 14 (1996), pp. 4129-4133. Such additive is called an internal releasing agent. However, the internal releasing agent may influence embossing condition as well as subsequent pattern transfer process. One of the examples of such concerns is cited in Japanese Patent PAJ-09099441 by Amatami et al., entitled xe2x80x9cInternal release agent for molding plastic lens and production of plastic lens,xe2x80x9d issued Apr. 15, 1997. In this document, polymer is molded by a glass mold to make a lens. Amatami et al. reported that the internal releasing agents caused color change, haze generation, and so forth. The haze appearance is due to light scattering by surface roughening. Haze is a big concern in fine patterning because the surface roughness would be equal to or larger than the patterned feature. Therefore, haze must be avoided in fine patterning.
The second approach for avoiding sticking between the mold and polymer involves modifying the mold surface. The surface of the mold can be modified by applying a releasing agent on it or by texturing it. However, surface texturing is not suited for embossing fine patterns because the texture features would be large in comparison to the pattern and would be transferred to the polymer. Thus what is needed is an effective releasing agent for embossing mold with high pattern density.
U.S. Pat. No. 5,861,113 issued to Choquette et al. discloses using fluorinated silane as a release layer against a curable plastic material. The fluorinated silane contains at least one alkoxy or halide group which is bonded directly to the Si atom of the silane and capable of reacting with oxide groups on a substrate or metal layer on the substrate. The fluorinated silane also contains at least one fluorinated alkyl group bonded to the Si atom of the silane. Examples of fluorinated silanes include tridecafluoro-1,1,2,2-tetrahydrooctyl-1-trichlorosilane from Petrarch Systems, Bristol, Penn. A thin layer of the fluorinated silane is formed on a grating pattern on a surface of a substrate using vapor deposition techniques. The silane groups of the fluorinated silane react with oxide groups on the substrate surface to provide a relatively strong covalent bond of the release layer to the substrate. Embossed gratings are fabricated on a waveguide surface by placing a small drop of UV curable epoxy on the waveguide surface and pressing the epoxy drop onto the silanized grating pattern. The epoxy is then cured, and the waveguide surface is removed from the silanized grating pattern, leaving an embossed grating on the waveguide surface.
In one aspect, the invention relates to a releasing agent for embossing a film carried on a surface of a substrate with a mold having a pattern which comprises an alkyl silane. In one embodiment, the alkyl silane forms a self-assembled monolayer on a surface of the mold.
In another aspect, the invention relates to a method for forming a pattern in a film carried on a substrate which comprises depositing a layer of alkyl silane on a surface of the mold having a pattern, bringing the method in contact with the film on the substrate and pressing the mold into the film so as to emboss the pattern on the mold in the film, and removing the mold from the film. In one embodiment, a thickness of the alkyl silane layer is molecular monolayer.
Other aspects and advantages of the invention will be apparent from the following description and the appended claims.